[request] Make sure set_message() actually displays its given message (closes #3003425)
The default automatic form sets a standard submit message as session
data. However in some cases, such as the cancellation of a form edition,
another message is intended to be printed on-screen.
This patch makes sure that set_message() really scrubs all previous
messages (and ids) from the request object and from the session data.
<table class="attributeForm" style="width:100%;"
tal:attributes="id tab_id | nothing;
class tab_class | nothing;">
<tr tal:iter="widget lines">
<th class="labelCol" tal:content="structure python:widget.render_label(entity)">attrname</th>
<td tal:define="error python:widget.render_error(entity)" style="width:100%;"
tal:attributes="class python:error and 'error' or nothing">
<div tal:replace="structure error">error message if any</div>
<div tal:replace="structure python:widget.edit_render(entity)" >widget (input, textarea, etc.)</div>
<div tal:replace="structure python:widget.render_help(entity)">format help if any</div>
</td>
</tr>
</table>